1. Field of the Invention
The present invention relates to an imaging lens to be mounted on an imaging apparatus, e.g. a digital still camera, a camera-equipped cellular phone or a personal digital assistant (PDA), that uses an imaging device, such as a CCD (charge coupled device) or a CMOS (complementary metal oxide semiconductor).
2. Description of the Related Art
Recently, size reduction and pixel-density increase have advanced conspicuously for the imaging devices, such as of CCDs and CMOSs. This in turn requires an imager proper and a lens for mount thereon, that are smaller in size but higher in performance. For size reduction, there is a need to reduce the overall length and the diameter (diametrical size reduction orthogonal to the optical axis). Meanwhile, besides size reduction, the imaging optical system generally is required to have a telecentricity, i.e. characteristic to place the main rays of light incident upon the imaging device at an angle nearly parallel with the optical axis (incident angle upon the image plane nearly zero relative to the normal line to the image plane). In order to secure a telecentricity, it is advantageous to arrange an optical aperture stop in a position as close as possible to the object. JP-A-2005-292235 discloses an imaging lens having totally three lenses wherein its optical aperture stop is arranged in a position closest to the object. Meanwhile, JP-A-2004-302058 discloses an imaging lens having totally three lenses wherein its optical aperture stop is arranged at between the first lens and the second lens.
In the meanwhile, as pixel-density increases for the imager, there is a trend required for the imager for taking a still image to provide a mechanical shutter in order to reduce signal noises at the imager. Where providing a shutter, it is advantageous to arrange it in a position close to the optical aperture stop in order to reduce the unevenness of light amount. Meanwhile, for an imaging lens arranged with three lenses, the optical aperture stop is advantageously arranged in a position as close as possible to the object, e.g. in a position in front or back of the first lens, in order to secure a telecentricity as noted above. However, it is unadvantageous to arrange the shutter mechanism in front of the first lens or closest to the object, in respect of size reduction. For this reason, it can be contemplated to provide a shutter mechanism in a position inside the lens system, i.e. at between the first and second lenses. For this purpose, there is a desire to develop a lens having a high aberrational performance compatible with pixel-density increase while securing sufficiently an air-spacing at between the first and second lenses, in order to arrange a shutter mechanism in an imaging lens arranged with three lenses. JP-A-2005-292235 includes example 3 that a spacing is secured comparatively broad at between the first and second lenses, thus providing a lens arrangement advantageous in arranging a shutter mechanism. However, there is a desire for developing a lens which is more advantageous in shutter mechanism arrangement and higher in aberrational performance than that structure.